The genetic affinities and evolutionary histories of most of the organisms of Earth, including Man and the other mammals, are uncertain or unknown. The fossil record of mammals is good, but incomplete, and studies of comparative anatomy and amino acid sequences have produced conflicting evidence and raised new questions. The principal objective of this application is to compare the base sequence complementarity of the single-copy nuclear DNAs among species of mammals using the technique of DNA-DNA hybridization, which provides a measure of median DNA sequence divergence. Available DNA sequence data are congruent with DNA hybridization evidence. Thus, the two methods give the same answer to the kinds of questions posed in this study, but DNA sequences yield gene phylogenies; DNA hybridization produces organism phylogenies. DNA hybridization averages the entire genome and is faster than sequencing, at least for the foreseeable future. This study proposes to determine the rate(s) of DNA evolution and the order of branching of mammalian lineages, especially for the primates, ungulates, and rodents. It has the following specific aims: 1. To continue a study of the primates by comparing the scnDNAs of the non-hominoids (monkeys, prosimians, tarsiers), including comparisons with the DNAs of Man, the other hominoids, and the next nearest relatives of the primates. 2. To continue a study of the rodents. Rodent scnDNAs are known to be evolving 5-10x as fast as those of the hominoids, but only the smaller species have been studied. 3. To initiate a study of the ungulates (horses, swine, hippos, rhinos, camels, giraffe, deer, cattle, antelopes) whose good fossil record should permit calculation of rates and dates in absolute time. 4. To identify and assess the factors that determine, influence, or are correlated with, the rate(s) of genomic evolution in mammals. These include the effects of age at first breeding, generation time, germline cell cycles, longevity, body size, litter size, neutral mutation rate, DNA repair efficiency, and possibly others. 5. To reconstruct the branching pattern and measure median genomic rates of evolution of the major mammalian lineages.